This invention is concerned with focal plane array technology for detecting incoming electromagnetic radiation with a two dimensional array of detectors.
Many advanced optical sensors require spectral selectivity as part of the function of target detection and discrimination. In the infrared region, for example, various methods have been proposed to provide multicolor infrared detection, some making use of a complex detector architecture, others using spectral filtering methods. Multiple apertures, or the use of dichroic beamsplitters with separate focal planes, are among the optical solutions which have been devised. Alternatively, segmented filters are placed in close proximity to the focal plane. This last approach has the disadvantages of complicating the optical and mechanical design, increasing the cooling requirements, and introducing the possibility of spectral crosstalk. Current technology requires that such filters be deposited onto separate substrates because most optical thin films must be deposited onto heated substrates at temperatures well above the safe limits for HgCdTe detectors, while filters deposited prior to detector formation must withstand the subsequent handling procedures. Filters on separate substrates incur several system penalties. The filter is a separate component which must be cryogenically cooled, placing increased demands on system cooling capacity. Furthermore, multicolor operation requires the use of a segmented focal plane with scanning capability to obtain accurate spatial as well as spectral information.
Spectral filter arrays are known in the art for visible wavelength applications. These filters consist of arrays of thin film interference filters, each color being determined by a totally independent filter design. This approach cannot be applied to an integrated multicolor filter because different color filters are made by changing the thickness of each of the separate coating layers in proportion to the ratio of the two wavelengths desired. A criterion for a suitable filter design for infrared applications is that the wavelength be selectable by modification of one or, at most, a small number of layers within the overall structure. Adhering to this constraint makes the fabrication of a patterned array more feasible because two completely independent filter designs do not need to be deposited.
As a consequence of these restrictions, current infrared focal plane array technology is limited to a single wavelength band of operation for each detector array or subarray. Multicolor focal planes require separate staring arrays with individual spectral filters or are built up of individual linear array components (usually several elements in width), each associated with a different bandpass filter. Other multicolor approaches require scanning of the scene over multiple linear arrays with individual filter elements.
An improved optical thin film technology which would integrate spectral bandpass filters with a focal plane array could have a major impact on future imaging and surveillance systems. New material and processing technology is needed to permit the deposition of spectral filters onto detector arrays at ambient temperatures and to fabricate filter arrays directly on the focal plane arrays so that different regions of the same array can respond to different wavelengths.